The present invention relates to a switching power supply suitable for use, for example, with a television receiver. More particularly, the invention relates to a resonant-type of switching power supply which can effect efficient power conversion with a simple configuration.
Switching power supplies for use with televisions, computer monitors, and the like, are typically characterized as xe2x80x9csoft switchingxe2x80x9d, which designates operation with a sine wave, or xe2x80x9chard switchingxe2x80x9d designating operation with a rectangular wave. The soft switching power supplies are considered to be superior in terms of power conversion efficiency, noise level, cost, and so forth. A large screen television typically requires a switching power supply capable of operation with AC input voltage of 100V and maximum load power of 160 W.
Prior art soft switching power supplies are described, for example, in an article entitled xe2x80x9cA New Magnetic Flux Control SMPS and the Multiscan Deflection Systemxe2x80x9d, by Masayuki Yasumura et al., IEEE International Conference on Consumer Electronics, Jun. 4, 1986. The switching power supplies described in that article contain four basic components: a power regulating transformer (PRT), a power isolation transformer (PIT), a converter drive transformer (CDT), and a resonant switching element. The characteristics of the PRT may be controlled by a feedback signal based on the measured output voltage, so as to adjust the switching frequency of the switching element, which in turn results in the output voltage being modified. In this manner, the output voltage is maintained in a desired range.
Present day switching power supplies of the self-excited oscillation type often have the following disadvantage: there is the possibility that a high voltage may be applied to the switching transistor, which requires the switching transistor to have a high voltage withstanding property, e.g., 1800V. Consequently, this property limits the ability to increase the switching frequency of the device because the power conversion efficiency is deteriorated by an increase of the power loss when the switching frequency is increased beyond a certain level.
In other configurations designed to alleviate this problem, universal switching transistors with lower voltage withstanding properties have been used, e.g., 1500V. However, these configurations are of increased complexity and size, requiring typically four large switching transistors and isolated signal transmission means such as a photo-coupler to keep the primary and secondary sides isolated from one another.
Accordingly, a need exists for a switching power supply capable of using a relatively low voltage withstanding transistor(s) and having a simple configuration.
It is therefore an object of the present invention to provide a switching power supply that uses a switching element of a desired voltage withstanding property, and which achieves a desired AC/DC conversion output.
Another object of the present invention is to provide a switching power supply with a simple configuration.
In accordance with the invention, there is provided a power supply including a converter transformer having primary and secondary windings loosely coupled to one another, and an auxiliary winding with a predetermined number of turns wound contiguously from one end of the primary winding to an end portion of the converter transformer. An input voltage smoothing capacitor is connected on one side to an end of the primary winding of the converter transformer through a forwardly directed diode. A switching element is connected to the other end of the primary winding. A control means controls the switching element to perform switching so that an output voltage of the secondary winding has a predetermined voltage value (or is within a predetermined voltage range). A second capacitor is coupled across opposite ends of the auxiliary winding via the forwardly directed diode, to boost-up voltage applied to the converter transformer.
The converter transformer may be comprised of an EE-shaped ferrite core having a pair of middle magnetic legs shorter than two pairs of outer magnetic legs thereof such that a gap is formed between the pair of middle magnetic legs. The primary winding is wound around one of the middle magnetic legs, the secondary winding is wound around the other of the middle magnetic legs, so as to provide the loose coupling between said primary and secondary windings.
The control means may be formed as a self-excited oscillation switching frequency control means for varying an inductance of a control transformer with an output voltage of the secondary winding to control an oscillation frequency. Alternatively, the control means may be formed as a separately excited oscillation switching frequency control means for detecting the output voltage of the secondary winding and controlling an oscillation frequency of an oscillation circuit with the detected value.
Advantageously, with the power supply of the present invention, the switching frequency can be controlled by separately excited oscillation, the voltage to be applied to the primary winding can be boosted up, and the voltage to be applied to the switching element can be set arbitrarily. Consequently, a desired output can be obtained using a switching element of a desired voltage withstanding property and a device having superior characteristics can be used to achieve efficient power conversion. Further, the number of parts can be reduced to reduce the area of a circuit board thereby to miniaturize the entire apparatus.